I'd be looking for something mid-range, from the very early 1990s. That typically gives Shimano 105SC or 600 Ultegra components and a CrMo butted frameset. You'll get excellent shifting performance via SIS indexed shifting combined with the noched and ramped HyperGlide cogs. You'll get 7 or 8 cogs, depending on the exact year. The freehubs build strong wheels and by that time 32 spoke was pretty much standard, so you'll have a bit less air rersistance for triathlons. Shimano brakes were SLR by that time, so there's good modulation and stopping performance. Down shifters were still available so you don't have the extra expense of brifters (shifters integrated into the brake levers) and can mount tham directly to triathlon bars, for that last bit of aero advantage. Lastly, compatible replacements for this era, particularly the HyperGlide cogs, are still readily available.